Apparatus for receiving and conditioning organic waste by anaerobic bioconversion

ABSTRACT

A tank (1) for an apparatus for receiving and conditioning organic waste by anaerobic bioconversion, in particular, waste produced by restaurant kitchens and other facilities, includes a main enclosure wherein bioconversion takes place, and a secondary enclosure (18) for receiving and storing ground organic waste before it is transferred to the main enclosure for completion of its bioconversion. A hopper (4) or other device for receiving the organic waste is associated with a grinder (2) and is connected to the secondary enclosure (18) of the tank for feeding the ground organic waste. A recirculation system (12, 15) for recycling the contents of the tank includes a pump (13), means for distributing (17a, 17b) the contents of the main enclosure between the different levels thereof, and separate means (9, 22) for removing solid residues and liquid waste.

FIELD OF THE INVENTION

The invention relates to an apparatus for receiving and conditioningorganic waste that operates by anaerobic bioconversion, as well as aprocess for treating a flux of organic waste in this apparatus. Theprincipal application of the invention is the treatment of wasteproduced by restaurant kitchens and other facilities, but otherapplications are also possible.

BACKGROUND OF THE INVENTION

The reduction and elimination of, or the recovery of energy from,organic waste from urban activities is a problem that can be approachedfrom several angles, the studies resulting therefrom leading to varyingsolutions according to the importance attached to the differentparameters under consideration.

Formerly, food waste was sold to serve as animal feed. Today, thistraditional mode of recycling food waste has given way to a policy ofdestruction, which is much more expensive. In parallel, there is agrowing tendency to increase the intermediate stockage time for wastefood, in order to allow more time between collection operations whosecost is also ever increasing.

The solutions imagined to date to combat the risks associated with foodwaste waiting for collection are refrigeration or partial sterilizationusing chemical products; both involve the temporary and costly slowingof an uncontrolled biological activity.

In contrast to procedures in use heretofore, the present inventionprovides for a local and immediate treatment of organic matter to carryout controlled biological processes in a specially designed digestor.

The present invention principally takes into account the mass of kitchenwaste to be managed by restaurants serving a large number of meals. Theinvention is derived from two considerations:

On the one hand, making available to these facilities an anerobicdigestor that, while being as compact as possible is able to receive themass of waste and continually restitute therefrom waste waters,concentrated solid residues and biogas of high-quality (from the pointof view of good calorific value and combustibility), responds to a realneed.

On the other hand, water purification stations, composting apparatus orother installations of the same type operate conventionally based on theprinciple that fluxes of organic waste are collected on as large a scaleas possible and delivered to a center where the bioconversion processtakes place in relatively stable conditions.

Stability is indeed an essential condition for maintaining thebiological process for transforming the materials.

SUMMARY OF THE INVENTION

Taking into account the flux of kitchen waste produced in restaurants orother facilities of the same type, the present invention departs fromthe principle set out above. It aims to install "digestors" at the verysites where the waste is produced and proposes a treatment system which,by carefully acting on the digestor's dynamics, enables thebioconversion process to be sustained, despite any temporaryquantitative or qualitative variations or irregularities in the flow ofwaste to be treated.

In this manner, the food waste will be treated before it has enough timeto develop any pathogenic activity. Thanks to the action ofmicro-organisms and the methanogenetic process, its potential energywill be converted to a valuable product, biogas.

The principal idea of the invention is to create an apparatus for thetreatment of organic waste that can be operated precisely and at will,so that the treatment process takes place as efficiently as possible,despite variations in the flux of waste to be treated.

To this end, the first object of the invention is an apparatus accordingto claim 1, and its second object a process according to claim 14.Further characteristics of the invention are set out in the dependentclaims.

The tank according to the invention comprises a main enclosure wherebioconversion takes place, and a secondary enclosure for receiving andstoring organic waste before it is transferred to the main enclosure tocomplete bioconversion.

As the organic waste is produced, for instance in a kitchen, it isintroduced into a receiving device, ground by means of a grinder orother mechanical disintegration means, and the ground waste is fed intothe tank's secondary enclosure.

The capacity of the secondary enclosure 18 can correspond at least tothe maximum quantity of ground waste treated per day. It is normallyfilled once per day and the daily mass of waste will remain in thesecondary enclosure for about a day. During its stay in the secondaryenclosure, the waste is pre-heated and begins a first phase of gasproduction, without releasing unpleasant odors. This ground waste in thesecondary enclosure forms a homogeneous mass, whose homogeneity can bemaintained or improved by recirculation or mixing by means ofrecirculation circuits described below.

Then, metered transfers of the contents of the secondary enclosure intothe main enclosure are made as a function of the progression ofbioconversion, by means of a system of circuits for recirculating thecontents of the main enclosure and the secondary enclosure.

The system of recirculation circuits includes a pump and means fordistributing the content of the main enclosure between different levelsthereof, which enables taking off and injecting amounts of the contentof the main enclosure at selected levels according to parameters of thebiomethanisation process.

The main enclosure of the tank where biomethanisation takes placecontains a stratified nonhomogeneous fluidic mass having a spongy layerfloating on its free surface, with an accumulation of dense sludge atthe bottom. By taking off and injecting amounts of this stratifiedfluidic mass at selected levels, it is possible to control theconditions of biomethanisation continuously and reliably.

The system of recirculation circuits also includes discrete means forextracting solid residues and spent liquids, enabling intermittentremoval of metered amounts of selected parts of the contents of the mainenclosure via extraction means where the solids and liquids arerecovered.

To maintain the contents of the main enclosure in equilibrium, eachamount removed via the extraction means will be followed by the transferof a more-or-less equivalent amount of ground waste from the secondaryenclosure into the main enclosure. This enables progressive feeding ofthe main enclosure with metered amounts of a pre-heated homogeneousmass, hence without any thermic shock liable to affect the equilibriumof the biomethanisation process which in particular requires greatthermic stability.

The secondary enclosure is preferably separated from the main enclosureby a separating partition located inside the tank, the main andsecondary enclosures being capped by a common top zone for thecollection of biogas. Advantageously, this partition includes aninclined upper portion enabling solid matter floating in the mainenclosure to overflow into the secondary enclosure, this inclinedportion of the partition being perforated to allow liquids to filterinto the main enclosure. In this manner, solid matter which has not beendigested during the biomethanisation process drops into the secondaryenclosure where it is mixed with the ground waste stored therein, tolater be recycled into the main enclosure.

As the biogas leaves the tank, by measuring its CO₂ /CH₄ ratio forinstance by spectrometric or calorimetric analysis of a burner flame, itis possible to evaluate the carbon/nitrogen ratio of the waste in thetank's main enclosure, thereby enabling actuation of an auxiliary feeddevice arranged to feed into the tank metered amounts of an additivematerial ("supplement"), in particular water, carbon-containing matteror nitrogen-containing matter. This device may for example include adevice for separating fat from fatty waste water, connected directly orindirectly to the secondary enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

We will now describe, by way of example, a main embodiment of thedigestor, and several possible variations, as well as a manner ofcarrying out the method. For this, reference is made to the drawings,wherein:

FIG. 1 is a simplified schematic representation of a digestor;

FIG. 2 is a simplified schematic representation of a separator that canbe associated with the digestor of FIG. 1; and

FIG. 3 is a schematic representation of a variation.

PREFERRED EMBODIMENTS

FIG. 1 shows the principal components of an embodiment of the digestor.It comprises a closed tank 1 wherein the anerobic bioconversion takesplace. This tank 1 may be made of stainless steel sheet, or of plasticmaterial. When for example the duration of the transformation cycle isabout 10 days, and the amount of waste to be treated is on average 25liters per day, the tank will have a capacity of about 500 liters atleast, so the level of the mass of waste can fluctuate withoutobstruction. The tank is generally cylindrical, with a dome on its upperpart to allow the accumulation of a gas phase, in principle a mixture ofmethane (CH₄) and carbon dioxide (CO₂), and a conical bottom at its baseto facilitate the evacuation and circulation of sludge.

A feed unit 2 is arranged to feed waste to be treated into the tank 1.Unit 2 is in the form of a grinder driven by a motor 3 and equipped witha feed hopper 4. The grinder drives the waste loaded into the hopper 4into the tank 1. As needed, the feed unit could have differentconstitutions. Several variations will be described later.

A separator unit 5 forms an additional enclosure connected to the tank1, arranged such that the principle of communicating vessels providesbalancing of levels between the tank and the separator, whereby thedifference of the levels between these two parts of the digestorcorresponds to the gas pressure in the tank. At its lower part, theseparator unit 5 includes a cyclone or centrifuge 6 which controlsconcentration of the solid phase that will be extracted at thislocation. In certain cases, this separation unit can be a simpledecanter.

Above the separator unit 5 extends a column 7 wherein the stabilisedliquid phase accumulates. At the top of this column is an overflowfunnel 8. The solid phase of residues separated in the unit 5 isextracted via a valve 9 and drops into a container 10 equipped withdraining means, for example exchangeable permeable bags that can befitted to allow draining of the residues and their removal. In the casewhere the member 6 is a centrifuge, the separator unit will be equippedwith an outlet 22 for the liquid phase and a flap (not shown) throughwhich the solid phase can be removed in the form of cylindricalbriquettes.

An important part of the described installation is the recirculationunit 11. This includes a multi-way distributor 12, a recirculation pump13 and various pipes interconnecting the digestor's components. First, aT-shaped pipe 14 interconnects the outlet of grinder 2, on the one handto the inlet of a secondary enclosure 18 inside the tank 1, and on theother hand to an inlet of distributor 12. It is hence possible to feedground waste into the secondary enclosure 18, where the waste can remainwaiting in the presence of gas already formed by preheating andhomogenization, to then be transferred as metered amounts ofhomogeneized and preheated mass of waste into the principal enclosurevia the distributor 12, pump 13 and recirculation circuit 15.

The recirculation circuit 15 constitutes means by which thebiomethanisation process can be precisely controlled. It is a system ofpiping provided with the necessary valves and notably with controlmembers 17a and 17b. In the example of FIG. 1, it consists of thefollowing:

Two piping segments 16a and 16b connect respectively the bottom of thetank 1 to the multi-way valve 12, and the latter to the intake of pump13. The section 23 of circuit 15 connects the outlet of pump 13 on theone hand to the inlet of member 17a, and on the other hand to anoverflow tap in the overflow funnel 8. Lastly, connector sections 24 and25 connect the separator unit's cyclone or centrifuge 6 on the one handto piping 16a and on the other hand to distributor 12.

The members 17a and 17b, which are parallel to one another, plunge intothe tank 1. They each have the form of a rod-like assembly formed ofconcentric tubes one of which can be rotated by means of a motorsituated at the top of the member. These tubes are provided withcoinciding openings at different levels so that, depending on therelative orientation of the tubes, the fluid in the inner tube can passout into the tank at a given level, or fluid in the tank can pass intothe tube, also at the desired level. The rod-like assembly 17b isconnected to the multi-way distributor 12 by a connexion 17c. Hence,thanks to the distributor assembly 12, 17a, 17b, the circuit 15 enablesfluid in the tank to be taken off at a desired level, to be delivered tothe output unit 5, or to be recirculated at any given level in the tank.

The inner partition 18a separating the auxiliary enclosure 18 from theremainder of the tank includes an upper part formed of an inclined flatplate provided with an opening 19. A lower part in the form of a tube ofrelatively large diameter extends down below the secondary enclosure 18where the grinder 2's outlet, leading into the pipe 14, penetrates intothe tank.

Thus, the digestor's main enclosure and secondary enclosure 18 can befed, mixed, interconnected and emptied by the described recirculationsystem.

Lastly, the tank's dome is connected by an extractor piping 20 to aprocessing unit 21 supplying biogas ready for later use. Depending onthe size of the envisaged application, i.e. the quantity of waste to betreated, biogas, which is nothing but a renewable form of natural gas,will be delivered to a water heater, or to a heat-exchange unit.

The unit 21 comprises a water separator, a safety valve and a flowmeter. A hydraulic valve having a water column of given adjustableheight, through which the gas passes by lubrication, is also integratedinto the unit 21. This valve regulates the pressure in the tank 1. Ifneeded, a means for testing the combustibility of the gas can also beprovided.

Division of the digestor into a feed unit 2, a recirculation unit 11, aseparation and extraction unit 5, and a gas processing unit 21 allows aflexible and efficient control of the digestor as a function of threecriteria:

Physical criteria: formation of well separated solid, liquid and gasphases;

Chemical criteria: production of combustible gas providing anappreciable saving of energy; and

Biological criteria: sustainment of efficient life conditions forbacterial species.

From the structural point of view, a particularly compact arrangement isobtained by positioning the entire recirculation unit 11 below the tank1, in the form of a tank-supporting base member. The entire unit can bearranged inside a cabinet-like container, wherein the input hopper 4 isfor example accessible behind a flap-door like those fitted on certaingarbage containers.

As a variation, the tank 1 and the extractor means are enclosed in aclosed ventilated volume, such as a cabinet, and the waste-receivingdevice, for instance a container with a flap-door as well as the grinder2, are arranged outside this cabinet at a location closer to where thewaste is produced, and are connected to the tank 1 by a flexible tubing.For example, such an arrangement would enable the waste-receiving deviceto be located in a kitchen, whereas the cabinet/tank is situated in anadjacent place.

To avoid bad smells escaping or risks in case of gas leakage, the upperpart of the housing forms a compartment fitted with means forventilating the entire apparatus.

Auxiliary devices to be provided include instruments for measuringimportant parameters: pressure, temperature, level, pump and grinderflowrates and, possibly, the pH. They also include control meansenabling actuation at will of the elements of the distributor assembly,pump 13, separator 6, etc, as well as of heating bodies for maintainingthe tank at the desirable temperature (35 to 55° C.).

The digestor as described above can operate efficiently in numerousdifferent conditions. The feed unit 2 can include, in addition to thegrinder, a pump outputting a mixture of preconditioned waste into thepipe 14. The pressure in the tank 1 is adjusted by the unit 21 asindicated above, and a siphon means 34 automatically removes the surplusof fluid waste accumulating in the enclosure 18. This surplus drops intothe input hopper 4 of the shredder or grinder, providing a signal topersonnel who feed waste into the apparatus that the apparatus ismomentarily saturated.

As a variation, it is also possible to provide an arrangement whereinthe supply unit is replaced by a two-level device whose hopper suppliesa centrifugal shredder able to deliver the mass of waste into asecondary compartment arranged at the top, exposed to the air. Thiscompartment can be fitted with an overflow and constitutes, with theinput hopper, a closed circuit enabling the ground waste to becirculated/mixed while it is waiting to be delivered by the grinder 2 tothe tank's enclosure 18. With such an arrangement, it has been observedthat during a waiting period a first aerobic heat-producingbioconversion takes place in the mass of waste exposed to the air.

The described construction can be designed in modular form. If required,several tanks 1 of the same dimensions can be connected in parallel toincrease the capacity. A modular construction enables digestors to bearranged in an optimal manner, adapted from case to case to theparticular treatment conditions.

As for all foodstuffs, kitchen waste consists principally of quatenaryproducts containing the elements carbon, hydrogen, oxygen and nitrogen,or ternary products containing carbon, hydrogen and oxygen. However, fora proper maintenance of bacterial species in a digestor, the C/N ratioshould be controlled to maintain it at a suitable value.

Kitchen waste from a restaurant represents a flux of matter which isbasically variable in quantity and composition.

The operation of the described system thus implies knowlege of thevariations to be expected in each particular case, which enables acontrol program for the digestor to be established.

The restaurant personnel will be instructed to tip the waste into thehopper 4 progressively as the waste is produced. The grinder 2 can beautomatically controlled, for example in response to the detection ofthe presence of a mass in the hopper 4. Its control can also be manualor by timer.

As indicated above, the ground flux leaving the grinder 2 is stored forpreheating and homogenization during a waiting period in the secondaryenclosure 18. In certain cases, control of the process can be entitrelyautomated. At each moment, the instantaneous state of the process isrepresented by a set of data supplied by measuring means. A comparisonof this data with predetermined threshold values enables a determinationof when and how the extraction means and/or the recirculation meansshould be actuated, possibly also the heating means and the auxiliaryfeeding means.

The introduction of ground kitchen waste into the main enclosure of atank as described creates a fluid mass that has a tendency to stratify.At the free surface, a layer of spongy or emulsified structure isformed, whereas dense sludge accumulates at the bottom of the tank. Thebiochemical process takes place in a median zone, with release of gasbubbles which collect in the upper common zone belonging to the main andsecondary enclosures, in which zone the gas-releasing potential createsa given pressure inside the tank.

The stabilized liquid phase will be extracted via the piping 22 andremoved via waste water drainage system. The solid phase usually hasonly a very small volume. It can be compacted and removed using a bagplaced in the container 10.

As regards the stabilized gas phase, to a certain extent its compositiondepends on how the bioconversion is controlled. This combustible gas canbe collected and stored for subsequent use. However, it can also beimmediately used in situ by feeding it to a water heater burner, forexample a condensation-type water heater including recovery of thelatent heat of the combustion gases. The hot water produced can be usedto maintain the tank hot, or for any other purpose. This use has theadvantage of considerably simplifying the gathering of data relative tothe composition and characteristics of the biogas produced in thedigestor, because analysis of the burner flame, for instance by opticalmeans, immediately provides the required data.

The control of the process includes metered transfers of temporarilystored matter from the enclosure 18 into the main enclosure, possiblywith the addition of supplements, and the recirculation of givenportions of the fluid mass in the main enclosure, by taking off a massat a given level and reinjecting it at another level. Thanks to themeans 17a, 17b, this recirculation takes place without undue disturbanceof the zone wherein the biological activity is taking place.

The level of the fluid mass in the main enclosure can rise up to theopening 19 and overflow into the secondary enclosure. Preferably, theoblique portion of partition 18a has a grid structure, which enablesfiltration: the liquid phase returns back into the main enclosure,whereas the non-digested solids from the main enclosure drop into thesecondary enclosure 18 and are mixed with the temporarily-stored groundwaste.

All of the monitoring operations can if required be remote controlledfrom a control station by means of telemeasurement and telecontroldevices. In certain cases, where the conditions for supplying waste aresufficiently well established, control programs can be set up.

In cases where it is desired to treat waste from relatively largerestaurant kitchens, for instance say those serving on average more than300 meals per day, the above-described compact construction canunexpectedly result in a significant simplification in the usualwater-treatment installations and operations.

It is known that the elimination of fatty waste water is a burdensometask in restaurants of a given size. To avoid fouling of the piping, theinstallations must include a device for separating fats from liquidsthat can be removed with waste waters. These separators generallyinclude two chambers, one to retain/decant heavy particles, and one forflottation of fatty matter. Periodically, they have to be emptied andcleaned out, these operations unfortunately generating foul smells.

However, the digestor as described above can easily be associated with afatty matter separator as shown in FIG. 2. In this way, the treatment offatty waste water is dissociated with operation of the digestor, and iscarried out much more conveniently.

A separate tank 26 is provided for the flottation of fatty matter.Residual water from dish washing, by hand or by machine, is deliveredinto a receiving basin 27 from where it flows out into a grid separator28 where solid matter is removed and delivered to hopper 4. The fattywaste waters then go into an assisted flottation tank 26 wherein astirrer 29 turns them at controlled rate. An evacuator means 30, rotatedby the same motor as the stirrer 29, turns at the level of the liquid inthe tank, where floating fatty matter accumulates. An extractor 31delivers it to the hopper 4 (and from there to the digestor's enclosure18), whereas the waste water is removed either from the bottom of theflottation tank, or via piping (not shown) connected to the body of theflottation tank.

Two auxiliary devices considerably improve the efficiency of thedescribed separator. These are a heat exchanger 32, which cools theseparator and thus recovers heat from the washing machine water, whilefacilitating solidification of the fatty matter, and a bubbling circuit33. The latter extends from a by-pass provided in the gas pipes at theoutlet of tank 1, and penetrates by a diffuser in the lower part of theflottation tank 26. A pump or compressor can be provided in this circuitto expel gas passing though the liquid mass of the fatty waste water inthe separator, which gas returns to the outlet piping 20. This operationwashes the gases, increases the proportion of methane (because CO₂ ismore soluble in water than CH₄), and contributes to a better separationof the fatty matters.

The residual water also benefits from the operation because its carboncontent is increased which enables it to be used, if wanted, as a carbonvector to promote vegetable growth. A bubbling circuit analogous to thejust-described one can also be connected to the anaerobicbiomethanisation tank, where it serves as a means for stirring and foractivation of degassing by coalescence.

The recovery of fatty matter as described above enables monitoring ofthe C/N ratio, because fatty matter essentially contains carbon andhydrogen. Metered addition of fatty material can hence re-establish theC/N ratio and consequently enable the digestor to be operated with anincreased efficiency.

Furthermore, another means can be implemented to monitor the C/N ratioin the described digestor, namely the supply of waste paper serviettesand napkins into the input hopper. These materials contain littlenitrogen, much carbon and fibres. Fibres help with the extraction andseparation of solid phases by a flocculant effect.

FIG. 3 illustrates a variation of the embodiment shown in FIG. 1.Similar or identical elements to those of the already-described digestorare shown, and will be designated by the same references.

Moreover, the digestor of FIG. 3 includes several additions andimprovements that will emerge from the following description.

The tank 1 has substantially the same construction as that of FIG. 1,with its secondary enclosure 18, separated from the main enclosure bypartition 18a with opening 19. The downwardly-extending lower part ofenclosure 18 is connected to the pipe 14. The upper part of partition18a includes a grid 18b whose purpose will be explained later.

The supply unit 2 comprises a collection receptacle 40 with a flap door41. Into the receptacle 40 lead the outlet of a siphon means 34 and theoutlet of an auxiliary feed device 59, illustrated here as a canisterprovided with an inlet member, circulation device or electromagnetictap. The motor-driven grinder 3 is housed in the base of container 40.

The main differences between the embodiment of FIG. 3 and the previouslydescribed embodiment reside in the structure of the distribution andrecirculation system. Here, the direction/speed of rotation of pump 13can be reversed, so the fluid can flow in either direction and atdifferent speeds. Also, the distributor member designated by reference12, which was associated with two liquid offtake and reinjection devices17a and 17b, is here composed of four members designated respectively by43, 44, 45 and 46. These four members are of similar construction: eachhave a fixed tube with openings 48 distributed along its length, arotatable tube inside the fixed tube, also provided with correspondingopenings, and an indexing device that can be controlled to selectivelybring each of the openings in the inside tube into coincidence with afixed opening.

The two distributor members 43 and 44 are connected by their fixed tubeto each of the pump 13's inlets. They replace member 12 of the firstembodiment. Members 45 and 46 (like the members 17a and 17b) arearranged vertically in the tank's main enclosure. Four devices 47 forindexing members 43 to 46 are connected to an automatic programmablecontrol unit API, to which are also connected data sensors monitoringthe operating conditions in the apparatus, as well as drive motors forthe pump 13, grinder 3 and centrifuge 5.

The upper fixed opening of member 46 is connected to a pipe 49 openingabove the secondary enclosure 18.

The fixed openings of members 43 and 44 are connected to variouscomponents of the distribution system. The openings of member 43 areconnected respectively to the grinder 3 for feeding waste into thetemporary storage enclosure 18, to the bottom of the tank for thedelivery of sludge to centrifuge 5, and to a fixed opening of member 46for the purpose of recirculation, to be explained later, a fourthopening being provided to connect tank 1 to a second servo-controlledstandby tank.

For member 44, the fixed openings are connected to the bottom of thesecondary enclosure via the waste-feed pipe 14, to the centrifuge 5 viapiping 25 and to a fixed opening of member 45 for recirculation; afourth opening is also provided for connection to a standby tank.

The control, monitoring and safety elements of the apparatus will now bedescribed.

The siphon means 34 here includes two vertical inlet tubes 50, 51, theformer in the main enclosure, the latter in the secondary enclosure.These tubes extend to the same height, which determines the tank'smaximum level of filling.

The two basic components of the processing unit 21 are a water separator52, also operating as a safety valve, and an expandable bellows 53forming both a buffer reservoir and a means for regulating pressure inthe tank and in the gas circuit.

The operation of valve 52 can be seen from the drawing: Drops of waterdriven along the pipe 20 drip into the deepest section of the valve,which leads into an enlarged section wherein the level is controlled byan overflow tube. The difference between the two levels corresponds tothe pressure set by the regulator bellows 53. The latter carries a load54 whose weight determines the pressure in the tank 1, pipes 20, etc.

The immediate use of the biogas produced in tank 1 is controlled byunits 55 and 56. Elements 57 and 58 are respectively a volumetricgas-flow meter and a water heater wherein thermal energy released bycombustion of the gas is transferred to a circuit supplying a heatingbody 60 of tank 1, and is connected to a general circuit provided forany other use of the hot water produced. In unit 56 can be seen acirculator and a four-way valve controlled by the automatic programmablecontrol unit API, for selecting the functions of the water heater.

Two sensors provide for keeping record of the maximum and minimumexpansion states of the bellows reservoir 53 and for switching off thewater heater 58 when necessary.

The counter 57 and water heater 58 are also connected to the automaticprogrammable control unit API to ensure qualitative control of thebiomethanisation. For this, two different means are available, andelements required for at least one of these two control means areprovided. One of these means consists of permanently recording thevolumetric flow of gas and the thermic balance of the water heater. Thisenables calculation of the thermic value of the burnt gas from which itscomposition is known, at least approximately. It is known that biogasproduced in an installation such as the described digestor isessentially a mixture of non-combustible carbon dioxide, and methanewhose calorific value is known. The composition of the gas produced inthe digestor at a given moment provides an indication regarding theconditions of the biomethanisation reaction. This indication,interpreted as a function of the momentaneous state of the mechanicalmembers of the apparatus, enables the process to be controlled.

The other means for analysing the produced gas is optical observation ofthe burner flame. The composition of the gas can also be deduced fromthe flame's color, or more precisely, the magnitude of the spectralintensity of its radiation.

The method, which is also an aspect of the invention, is made up of thecombination of essential operations that must be carried out for thedigestor to operate as efficiently as possible, given the rhythm offeeding of the organic waste and the nature of the organic waste pouredinto the container 40. The conditions of biomethanisation should bemaintained as stable as possible in the active zone of the tank 1; gasshould be allowed to accumulate as regularly as possible in the piping20 and the buffer reservoir 53; and liquid rejects and solid residuesshould be regularly extracted from the tank. The operations to becarried out are divided into routine operations that are normallyrepeated at regular intervals; occasional operations repeated at arhythm depending on various factors, and that should be controlled incase certain conditions prevail; and safeguard operations to be carriedout to safeguard the life and the activity of the bacterial speciesactive within the digestor, this being a priority.

The first category of operations includes:

a) Introduction of matter into the secondary enclosure 18. As mentionedpreviously, the grinder 3 and pump 13 act to pump the organic matterfrom container 40 via pipe 14 into enclosure 18. This operation can becontrolled: by the personnel who empty the waste into the container;automatically; as a function of time; or as a function of the degree offilling of the container. Typically, the filling operation will becarried out at least once a day and the minimum capacity of thesecondary enclosure 18 will correspond to the maximum daily amount ofwaste previewed for the installation.

b) The extraction of stabilized liquid removed from the main enclosureof the tank, at a height situated below the floating top-layer. Thisoperation will be programmed so it is carried out as regularly aspossible, for example at a rhythm of 5% of the average daily amount fedinto the tank, every hour. The liquid is taken off via one of theopenings of member 46, and expelled by pump 13 via piping 25 andoverflow 22.

c) Immediately after each operation b), the same quantity of matter willbe extracted from the secondary enclosure 18 via pipe 14 and injected ata selected level in the tank 1 by member 46, in order to reestablish thelevel to which tank 1 is filled.

The indicated rhythm for operations b) and c) will be modulatedaccording to the data gathered. A total daily value greater or less thanthe fed quantity will tend to shorten or lengthen the residence time ofthe matter in the tank. This parameter will be factored into programmingof the operation.

d) Lastly, another routine operation which is periodically controlled(especially in the case where feed from the container 40 to theenclosure 18 takes place relatively frequently) consists of turning overthe structure of the piled matter temporarily stored in the enclosure18. The most recently added portion of the content of this enclosurewill be intake via the pipe 14, pump 13 and member 46 so it passesthrough the overflow pipe 49 and is relocated to the upper part ofenclosure 18. During this operation, the liquid phase of the transportedmatter will be filtered by grid 18b and pass directly into the mainenclosure of tank 1.

The second category of operations (occasional operations) includes:

a) Mixing of the matter in tank 1.

By suitably indexing the two offtake and injection members 45 and 46,liquid can be intake from a desired level in the main enclosure of tank1 and, via distributor 44, pump 13 and distributor 43, be restitute atanother level without agitating the entire liquid mass, hence withoutdisturbing the active zone where the active bacterial populationundergoes bioconversion.

b) Spraying the floating top layer.

The same elements as above also enable liquid taken from a stabilizedzone to be poured onto the floating top layer to assist the integrationthereof into the process.

c) The removal of solids.

As a general rule, the production of compact solid matter representsonly a minor percentage of the treatment products. Sludge will beremoved from the tank's base, delivered to the centrifuge 5 and, aftercompacting, removed via the collector vessel 10.

d) Feeding a supplement.

Should the waste with which the digestor is being fed have a compositionwhich is systematically deficient from the point of view of a desirablechemical balance, the feeding of a supplement from the device 59 can beenvisaged according to a set program. As already mentioned, the mixturesof materials fed to the digestor should maintain an equilibrium betweennitrogen-based and carbon-based components, to produce biogas with agiven CO₂ /CH₄ ratio determining its energy potential. If the waste tobe treated is predominantly nitrogen-based, the container 59 couldconsequently contain a carbon-based product, for instance an oil, andthe control members will periodically inject a metered amount of thisproduct into the container 40.

The safeguard operations consist essentially in ensuring favorableparameters for development of the active populations, namely:

a) Firstly, the digester must in all circumstances maintain an eventemperature, which is achieved by the above-described operations. Thetemperature can from case to case be of the order of 35° C./55° C., butits variations should not exceed plus/minus 2° C. Means can be providedfor signalling inacceptable variations.

b) Secondly, it is necessary to maintain a constant gas pressure in thetank, as well as in the bellows-reservoir 53 and the water heaterburner, whatever may be the instantaneous composition of the biogas.This pressure is held constant by the load 54 of the bellows-reservoir53, and any excessive extension or contraction of this member ismonitored by sensors. Surplus gas can be removed by the cabinet'sventilation system, whereas a reduction of pressure can be compensatedby feeding an appropriate supplement.

c) Moreover, the syphon means 34 controls any overload of the tank 1,whereas the grid 18b filters floating solid matter that may overflowinto the secondary enclosure 18. A momentary overload is signalled by anoverflow via the siphon means 34; if this happens, feed of new organicmatter to the container 40 will be interrupted or reduced in order tore-establish normal functioning.

The above-described digestor has displayed a remarkable performance.With a feed of organic matter from a restaurant, the following figureshave been observed:

Mean quantity treated:--50 kg/day

Weight distribution of rejects:--gas 20%--liquids 65%--solids 15%

The liquid rejects contain at least 5 g/l of organic matter and can beremoved directly via the waste water disposal system. The volume of gasproduced at ambient temperature and pressure is 88 l/kg of waste. Thethermic output from the produced gas is 15500 Kcal/day.

What is claimed is:
 1. An apparatus for receiving and conditioningorganic waste that operates by anaerobic bioconversion including abiomethanization process, comprising a closed tank wherein bioconversiontakes place, provided at its upper part with means for removing biogasproduced, and provided with means for the discrete extraction of solidresidues and liquid residues, the tank including a main enclosurewherein the bioconversion takes place, and a secondary enclosure forreceiving and storing ground organic waste before the transfer thereofto the main enclosure as feed for bioconversion; the secondary enclosurebeing separated from the main enclosure by a partition located insidethe tank; the apparatus further including a device for receiving organicwaste, associated with a grinder or other mechanical disintegratingmeans, and connected to the secondary enclosure of the tank to feedground organic waste into the secondary enclosure; wherein:theseparating partition is arranged to allow the storage in the secondaryenclosure of a quantity of ground waste up to a variable level, and alsoto allow matter from the top of the main enclosure to overflow into thesecondary enclosure; the apparatus further comprises a system ofcircuits for recirculating the content of the main enclosure and thesecondary enclosure, comprising a pump enabling inter alia meteredtransfer of the content of the secondary enclosure into the mainenclosure, and also distribution of the content of the main enclosurebetween different levels therein, by distribution means; and wherein:the apparatus is arranged so the content of the secondary enclosure ispre-heated to permit said metered transfer of the content into the mainenclosure without a thermic shock liable to disturb the equilibrium ofthe biomethanisation process.
 2. An apparatus for receiving andconditioning organic waste that operates by anaerobic bioconversionincluding a biomethanization process, said apparatus comprising a closedtank, said closed tank comprising an upper part comprising means forremoving biogas, and means for discrete extraction of solid residues andliquid residues; a main enclosure wherein bioconversion takes place, anda secondary enclosure for receiving and storing ground organic waste fortransfer to the main enclosure as feed for bioconversion wherein thesecondary enclosure is separated from the main enclosure by a separatingpartition located inside the tank; a device for receiving organic wasteassociated with means for mechanically disintegrating organic wasteconnected to the secondary enclosure of the tank and means for feedingground organic waste into the secondary enclosure, wherein theseparating partition allows storage in the secondary enclosure of aquantity of ground waste up to a variable level, and also allows matterfrom the main enclosure to overflow into the secondary enclosure; asystem of recirculation circuits for recirculating the contents of themain enclosure and the contents of the secondary enclosure, said systemcomprising a pump enabling inter alia metered transfer of the contentsof the secondary enclosure into the main enclosure, and means fordistribution of the contents of the main enclosure between differentlevels, wherein the contents of the secondary enclosure is pre-heated topermit said metered transfer of said contents from the secondaryenclosure into the main enclosure without a thermic shock liable todisturb equilibrium of the biomethanisation process.
 3. The apparatusaccording to claim 2, wherein the device for receiving waste comprises amember selected from the group consisting of an input hopper and acontainer with a flap door.
 4. The apparatus according to claim 2,wherein the main enclosure and the secondary enclosure and theseparating partition are topped by a common top zone inside the tank forthe collection of biogas.
 5. The apparatus according to claim 4, whereinsaid separating partition comprises an included upper portion enablingsolid matter floating in the main enclosure to overflow as overflowingmatter into the secondary enclosure, and an inclined portion perforatedfor filtration of liquids from said overflowing matter.
 6. The apparatusaccording to claim 5, wherein the means for removing biogas comprises aprocessing unit fitted with means for monitoring parameters of biogas.7. The apparatus according to claim 6, wherein the means for removingbiogas are connected to a burner associated with a water heater, andwherein the processing unit comprises means for analyzing a burnerflame.
 8. The apparatus according to claim 2, wherein said means fordistribution comprises circuits selected from the group consisting of atleast one of an offtake circuit and a reinjection circuit provided witha plurality of inlets and outlets situated at different levels of themain enclosure, said circuits being interconnected by the system ofrecirculation circuits and the pump; and wherein each said offtakecircuit and said reinjection circuit comprises an obturator means forselectively opening and closing each of the inlets and outlets of saidcircuits.
 9. The apparatus according to claim 2, wherein said means fordistribution comprises an overflow pipe arranged above the upper part ofthe secondary enclosure for circulating a most recently added part ofthe content of the secondary enclosure in order to turn over thestructure of the piled matter temporarily stored in the secondaryenclosure.
 10. The apparatus according to claim 2, comprising a set ofmeans for measuring and means for monitoring and means for controllingbioconversion, said means for controlling comprising at least one datasensor for at least one parameter selected from the group consisting ofpressure in the tank, level of the mass contained in the tank, and pH.11. The apparatus according to claim 2, comprising an auxiliary feeddevice for feeding a supplement to the tank, said supplement beingselected from the group consisting of water, a carbon-based product, anda nitrogen-based product.
 12. The apparatus according to claim 11,wherein the auxiliary feed device is arranged to meter the feed of thesupplement in response to a specific means for controlling.
 13. Theapparatus according to claim 12, wherein the auxiliary feed devicecomprises a device for separating fatty matter from fatty waste waterconnected to the secondary enclosure.
 14. The apparatus according toclaim 2, wherein the tank and the means for discrete extraction areenclosed in a closed ventilated volume.
 15. The apparatus according toclaim 14, wherein the receiving device and the grinder are arrangedinside the closed volume which is fitted with a door for feeding thewaste.
 16. The apparatus according to claim 14, wherein the receivingdevice and the grinder are arranged outside the closed volume at alocation which is closer to where the waste is produced, and areconnected to the tank by a flexible tubing.
 17. A method for treating aflux of organic waste in an apparatus according to claim 1, situated inthe proximity of a location where waste of a given category is produced,said method comprising:feeding waste into the receiving deviceprogressively as the waste is produced; grinding the waste to formground waste; feeding the ground waste into the secondary enclosure ofthe tank and, after a rest period; and feeding metered amounts of theground waste from the secondary enclosure into the main enclosure as afunction of progression of bioconversion.
 18. The method of claim 17,wherein said waste comprises kitchen waste.
 19. The method of claim 17,comprising taking off metered amounts of a selected part of the contentof the main enclosure, and feeding said metered amounts towards meansfor extracting solid and liquid rejects, said taking off said amountsbeing followed by a transfer of a substantially equivalent quantity fromthe secondary enclosure into the main enclosure.
 20. The method of claim17, comprising overflowing matter from the top of the main enclosureinto the secondary enclosure.
 21. The method of claim 19, comprisingrecirculating a most recently added part of the content of the secondaryenclosure as a recirculation content by said recirculation circuits andpouring said recirculated content onto the top of the secondaryenclosure in order to turn over structure of piled matter temporarilystored in the secondary enclosure.
 22. The method of claim 17,comprising monitoring a carbon/nitrogen ratio of the waste in the mainenclosure of the tank by measuring a CO₂ /CH₄ ratio of biogas outletfrom the means for extraction associated with the tank.
 23. The methodof claim 22, comprising feeding metered quantities of a member selectedfrom the group consisting of a strongly carbon-containing supplement anda nitrogen-containing supplement into the tank as a function of datasupplied by monitoring the carbon/nitrogen ratio, in a manner tomaintain the carbon/nitrogen ratio at an optimum value.
 24. The methodof claim 17, comprising combusting biogas at an outlet for heating aflow of water by released heat, evacuating combustion gas by ventilatinga closed volume, and analyzing combustion in a manner to provide datarepresenting a CO₂ /CH₄ ratio of the biogas serving as criteria forcontrolling the bioconversion.
 25. A method of using an apparatusaccording to claim 1 to treat kitchen waste.